The present invention relates to an engine igniting coil device and a method of winding a secondary coil of the device.
A secondary coil in a conventional engine ignition coil device is wound axially on a coil bobbin in such a manner that an element wire is wound in layers around sections of the coil bobbin, which sections are separated by a plurality of intermediate ribs and both end flanges. The coil bobbin has an increased number of sections separated by thick-wall ribs to assure necessary dielectric strength of coil turns laid in each section. Consequently, the conventional engine coil device using the above-mentioned type coil bobbin has a large size.
Japanese laid-open patent No. 60-107813 is directed to a compact engine ignition coil device which, as shown in FIG. 8, uses a no-ribbed coil bobbin 8' on which a coil wire 71 is wound axially in bank layers at a specified bank angle .theta. by a so-called bank-winding method permitting the setting of the dielectric strength of the coil interlayer insulation at a low value.
FIG. 10 depicts a conventional bank winding method by which a coil wire 29 being fed from a nozzle 30 reciprocating in the axial direction for a distance of a specified width w' corresponding to bank length l is wound axially in layers of wire turns one by one at a specified bank angle .theta. on a coil bobbin 8 which is rotated about its axis and, at the same time, moves axially.
The conventional bank winding method, however, involves a problem that the reciprocal movement of the nozzle 30 has its axis not parallel to the bank direction of wire turns and, therefore, causes a change in feeding rate of the wire 29 while the nozzle 30 moves from a position A to a position B, resulting in unevenness of the winding tension of wire turns on the coil bobbin.
In short, the conventional bank winding methods applied for manufacturing an engine ignition coil device has the following problems to be solved.
The first problem of the conventional bank winding method for axially winding a wire in banks of turns at a bank angle on a coil bobbin is that it is necessary to provide a sufficiently thick layer of insulating resin filled around the secondary coil to secure its dielectric strength according to potential distribution over the secondary coil wound on the coil bobbin.
This may present a particular severe condition for an open-magnetic-circuit-type engine igniting coil device which comprises a cylindrical coil case containing an ignition coil assembly integrally molded therein by potting with melted insulating resin and which is directly attached at its terminal to an ignition plug mounted in a cylindrical bore in a cylinder head portion of a vehicle engine. Namely, the ignition coil device must have a coil case of a diameter that is large enough to enclose the secondary coil of the assembly with a thick layer of insulating resin for assuring a sufficient dielectric strength.
The second problem is that a secondary coil formed on a coil bobbin 8', as shown in FIG. 8, by winding a wire 71 around a shaft of the coil bobbin 8' at a bank angle .theta. may be deformed due to a slip-down of banks of wire turns therein during and even after bank winding. Such slip-down in the secondary coil may result from the fact that several initial banks of wire turns could not be placed correctly at a given bank angle .theta. around the coil bobbin from the flanged portion thereof. A slip-down of any layer in the secondary coil causes an increase of a voltage between the layers of wire turns, resulting in a breakage of the interlayer insulation of the secondary coil.
The third problem is that the reciprocal movement of the wire feeding nozzle along an axis not parallel to an axis of bank direction causes a change in the feeding rate of the wire, i.e., a change of tension of the wire being wound during the nozzle movement, resulting in slip-down of the wire layers in the coil. Consequently, the thus formed secondary coil can not assure a constant dielectric strength of its interlayer insulation.